Patients with diabetes have to have the content of glucose in their blood measured frequently, several times per day for example. On the basis of this, a corresponding dose of insulin is administered. In doing so, with the aid of a needle the skin is punctured and the emerging blood is applied to a measuring strip as part of a measuring system. After the measurement has been obtained the insulin dose is calculated, wherein the insulin is injected by means of a further needle into a tissue of the patient. Several punctures of the skin are therefore necessary. In order to reduce the frequency of puncturing and the pain and discomforts associated therewith, a catheter can be implanted in the body over a longer period. Via this catheter (known as an indwelling catheter) a medication (e.g. insulin) can be administered, tissue fluid can be removed for glucose determination and/or by means of a sensor located on the catheter the glucose can be continuously measured.